dt-bindings: sun6i-dsi: Document A64 MIPI-DSI controller

The MIPI DSI controller in Allwinner A64 is similar to A33.

But unlike A33, A64 doesn't have DSI_SCLK gating so it is valid
to have separate compatible for A64 on the same driver.

DSI_SCLK uses mod clock-names on dt-bindings, so the same
is not required for A64.

On that note
- A64 require minimum of 1 clock like the bus clock
- A33 require minimum of 2 clocks like both bus, mod clocks

So, update dt-bindings so-that it can document both A33,
A64 bindings requirements.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191222132229.30276-2-jagan@amarulasolutions.com